The present invention relates to a system for guiding or transferring a beam of electromagnetic radiation, the system comprising planar media, and in particular a stack of dielectric media with a plurality of refractive indices and thicknesses. The electromagnetic radiation can be light, in which case the system is denoted an optical system.
Recently, resonant cavity light emitting diodes have attracted considerable interest, mainly because of the possibility of increased extraction efficiency as compared to standard LEDs. Record efficiencies as high as 20% have been reported in, for example, H. De Neve et al., xe2x80x9cRecycling of Guided Mode Light Emission in Planar Microcavity Light Emitting Diodesxe2x80x9d, Appl. Phys. Lett., vol. 70, no. 7, pp. 799-801, 1997. However, these devices exhibit a wide radiation pattem, making them less suited to fiber applications.
In the past, efforts have been undertaken to design resonant cavity light emitting diodes with a narrower radiation pattern, as explained for example in R. Bockstaele et al, xe2x80x9cResonant Cavity LED""s Optimized for Coupling to Polymer Optical Fibersxe2x80x9d, IEEE Phot. Techn. Lett., vol. 11, pp. 158-160, 1999. This approach involves growing an undertuned cavity, i.e. a cavity that is too short as compared to the resonance wavelength. This yields narrower radiation patterns, but unfortunately at the expense of lower extraction efficiencies since the microcavity resonance only enhances a limited subset of the spectral and angular spectrum emitted by the active layer.
It is an aim of the invention to provide a system and method for designing of resonant cavities with narrow radiation patterns.
The invented system guides a beam of electromagnetic radiation. The system comprises a stack of dielectric layers with a plurality of refractive indices and thicknesses of the layers. In the system, at least a first stack of dielectric layers with the following properties is disclosed. The first stack comprises at least a first substack, a second substack and a third substack, the third substack separating the first and second substack. The first substack comprises at least one dielectric layer, possibly a plurality of dielectric layers, the second substack comprises at least one dielectric layer, possibly a plurality of dielectric layers. Dielectric layers of the first substack and the second substack equidistant from the third substack have the same refractive index. Moreover, the sum of the thickness of dielectric layers of the first substack and the second substack equidistant from the third substack is a multiple of half of the vacuum wavelength of the beam divided by the refractive index of the dielectric layers of the first substack and the second substack having a same distance from the third substack. Further, the third substack thickness is substantially different from a quarter of the vacuum wavelength of the beam divided by the refractive index of the one dielectric layer of the third substack.
In a first aspect of the present invention, the first substack and the second substack each has an odd number of layers.
In a first embodiment of the first aspect of the present invention, the first substack and the second substack comprise dielectric layers with refractive indices n1 and n2.
In a second embodiment of the first aspect of the present invention, the refractive indices are substantially different such that the equivalent penetration depth of the beam is negative. The first substack can comprise a first and second layer with refractive index n2, a third layer with refractive index nl, the first and the third layer being attached to each other, the second and the third layer being attached to each other, and the first and the second layer not being attached to each other. Alternatively, the first substack can be a concatenation of a first layer with refractive index n2, a second layer with refractive index no and a third layer with refractive layer n2. In this case, the third substack can be a dielectric layer with index n1.
In a second aspect of the present invention, the first stack of dielectric layers is substantially transparent for normal incidence radiation and substantially reflective for off-axis incidence radiation.
In a third aspect of the present invention, the third substack comprises a periodic repetition of at least one of the stacks. This periodic repetition can be a periodic repetition of the first and the second stack, the refractive index of the dielectric layers possibly being different for at least two repetitions. This periodic repetition can also be a periodic repetition of the first stack. In this case, the refractive index of the dielectric layers can possibly be different for at least two repetitions.
In a fourth aspect of the present invention, the second stack of dielectric layers consists of a dielectric layer with refractive index n, and the thickness of the dielectric layer is a quarter of the vacuum wavelength of the beam divided by n1.
In a fifth aspect of the present invention, the third substack comprises a second stack of dielectric layers, the second stack of dielectric layers being substantially reflective for normal incidence radiation. The reflectivity of the second stack lies preferably within the range of 40 to 70%, advantageously being approximately 60%.
In another aspect of the present invention, the first stack of dielectric layers has a normal incidence matrix being substantially equal to the unit matrix.
In another aspect of the present invention, the third substack comprises a second stack of dielectric layers and at least one electromagnetic generating layer, the system having a narrow radiation pattern.
In another aspect of the present invention, at least one layer of the first or second stack has a thickness substantially different from a quarter of the vacuum wavelength of the beam divided by the refractive index of the layer of the first or second stack.
In another aspect of the present invention, the third substack consists of a single dielectric layer with a refractive index nC with a thickness equal to a multiple of half of the vacuum wavelength of the beam divided by nC.
Another aspect of the present invention relates to a method for generating a first beam of electromagnetic radiation with a narrow radiation pattern, comprising the steps:
providing a system according to the invention as described above,
generating a second beam of electromagnetic radiation with a wide radiation pattern;
letting the second beam resonate in between a first and a second structure; wherein the first structure being substantially transparent for normal incidence of the second beam on the first structure and substantially reflective for off-axis incidence of the second beam and the second structure being adapted for partly reflecting the second beam in a off-axis direction; and the part of the second beam being transferred through the first structure being the first beam.
Another aspect of the present invention relates to a method for generating a first beam of electromagnetic radiation with a narrow radiation pattern, comprising the steps:
providing a system according to the invention such as described above,
generating a second beam of electromagnetic radiation with a wide radiation pattern;
letting the second beam resonate in between a first and a second structure; wherein the first structure being substantially transparent for normal incidence of the second beam on the first structure and the first structure being adapted for having a negative equivalent penetration depth; and
the part of the second beam being transferred through the first structure being the first beam.
Another aspect of the present invention relates to a method for generating a first beam of electromagnetic radiation with a narrow radiation pattem, comprising the steps:
generating a second beam of electromagnetic radiation with a wide radiation pattern;
letting the second beam resonate in between a first and a second structure; wherein the first structure comprising a first substructure and a second substructure, the first substructure being substantially reflective for normal incidence for normal incidence of the second beam, the second substructure being substantially reflective for off-axis incidence of the second beam and substantially transparent for normal incidence of the second beam; and
the part of the second beam being transferred through the first structure being the first beam.